The present invention relates generally to wireless broadband communications systems, and more specifically to a system and method of providing time division duplex point-to-point radio links using orthogonal frequency division modulation that achieves increased reliability in environments in which portions of the available operating frequency band are subject to noise and/or interference.
Wireless broadband communications systems are known that employ orthogonal frequency division modulation (OFDM) to provide time division duplex (TDD) point-to-point radio links. Such wireless communications systems typically include at least one transmitter disposed at one end of a TDD point-to-point radio link, and at least one receiver disposed at the other end of the radio link. The transmitter may be configured to transmit data signals over one or more communications channels using specified error correction coding and modulation techniques. Further, the receiver may be configured to capture the transmitted data signals, and to employ specified signal processing techniques for decoding and demodulating the signals to recover the user data. In addition, such wireless communications systems typically employ adaptive modulation techniques to adjust various transmission parameters such as the coding rate and the modulation mode, thereby compensating for changes in channel characteristics that can adversely affect the quality and/or the rate of data transmission.
Wireless communications systems employing point-to-point radio links frequently operate in licensed frequency bands, which are generally allocated to assure that the systems are not adversely affected by interference caused by other systems operating in the same band or in different bands. For example, wireless communications systems operating in the same licensed band may be geographically separated from one another by distances sufficient to avoid interference from other systems operating in the same band. In addition, wireless communications systems in close proximity to one another may avoid interference by operating in different licensed bands. There are drawbacks, however, to operating wireless communications systems in licensed frequency bands. For example, users of such systems may be required to pay significant licensing fees to operate in the licensed bands. In addition, operating wireless communications systems in licensed frequency bands can sometimes result in inefficient use of the allocated frequency spectrum.
To avoid payment of high licensing fees, and to achieve more efficient use of the allocated spectrum, wireless communications systems may be configured to operate in shared frequency bands, such as the unlicensed frequency band for wireless LANs (WLANs) located at approximately 5.8 GHz. However, operating wireless communications systems in shared frequency bands also has drawbacks. For example, due to the unlicensed nature of the shared frequency band for WLANs located at 5.8 GHz, more than one WLAN operating in the shared band may be deployed within the same geographical area. Further, radar and/or other types of wireless communications systems, either operating in the same shared band or generating frequency harmonics having components in the shared band, may also be deployed within the same geographical area. As a result, wireless communications systems operating in shared frequency bands may be required to communicate over channels that are subject to increased levels of noise and/or interference, which can hinder the systems' ability to maintain high rates of data transmission.
Because wireless communications systems operating in shared frequency bands can be configured to follow specified frequency band plans and can employ specified bandwidths, the noise or interference generated by these systems may occupy just portions of the operating frequency bands available to other systems operating in the shared bands. As discussed above, wireless communications systems may employ adaptive modulation techniques to adjust the coding rate and modulation mode to provide the maximum payload consistent with reliable operation. However, adaptive modulation techniques typically adjust the coding rate and modulation mode based upon a measure of the received signal quality averaged over the entire operating frequency band. As a result, such systems may provide a reduced data rate, or may determine that a frequency band is unusable, even though noise or interference may be affecting only a portion of the operating frequency band.
It would therefore be desirable to have a wireless broadband communications system that provides increased reliability in environments in which portions of the available operating frequency band are subject to noise and/or interference, while avoiding the drawbacks of the above-described conventional systems.